fs/9p: remove unnecessary and overrestrictive check

fs/9p: remove unnecessary and overrestrictive check

This eliminates a check for shared that was overrestrictive and
prevented read-only mmaps when writeback caches weren't enabled.

Cc: [email protected]
Fixes: 1543b4c5071c ("fs/9p: remove writeback fid and fix per-file modes")
Reported-by: Robert Schwebel <[email protected]>
Closes: https://lore.kernel.org/v9fs/ZK25XZ%2BGpR3KHIB%[email protected]
Reviewed-by: Dominique Martinet <[email protected]>
Reviewed-by: Christian Schoenebeck <[email protected]>
Signed-off-by: Eric Van Hensbergen <[email protected]>